Research over the last decade has shown that appropriately timed bright light can shift human circadian rhythms. The findings from this research should be applied to the problems of shift work, jet-lag, and some sleep disorders. Bright light is also used to treat winter depression, and one theory is that it works by shifting circadian rhythms.Unfortunately, very little basic research has been performed that uses bright light to shift circadian rhythms. For example, very little is known about the parameters of bright light necessary to produce specific phase shifts. Furthermore, in practical applications, people living in the natural environment are exposed to 24-hour time cues that may prevent the shifting of their rhythms. The proposed field experiments will test ways to use controlled exposure to light to produce phase shifts in subjects who live at home. Most experiments include a simulated night shift and a shift in the sleep-wake schedule, as would be required by night work. Body temperature is continuously recorded as a marker of the endogenous circadian oscillator. Daily questionnaires are used to estimate sleep time and quality and subjective states (e.g., fatigue). Two experiments explore the duration of light pulses and the number of days of exposure needed to produce a 12-hour shift in the circadian rhythm of temperature. Another experiment explores the optimal timing of light pulses for producing a 9-hour shift. Another investigates whether it is better to shift the sleep-wake schedule gradually or abruptly to the new time while using bright light to shift circadian rhythms. Finally, an experiment explores what happens to circadian rhythms when the worker has to switch back to the day shift after having used light to successfully adapt to the night shift.